Bonding material and bonding method using the same

ABSTRACT

A bonded product is obtained by applying a silver paste containing silver nanoparticles having an average primary particle diameter of 1 to 200 nm, and performing firing. A diameter of a crystallite of the bonded product on a (111) plane of Ag when heated at 250° C. for 10 minutes in an inert atmosphere is 65 nm or larger.

The present application is a divisional of pending U.S. patentapplication Ser. No. 13/576,014, which is a National Phase ofPCT/JP2010/057293 filed Apr. 23, 2010 and claims the benefit of JapaneseApplication No. JP 2010-058370 filed Mar. 15, 2010. The disclosures ofU.S. patent application Ser. No. 13/576,014 and PCT/JP2010/057293 areincorporated by reference herein in their entireties.

FIELD

The present invention relates to a bonding material and a bonding methodusing the same.

BACKGROUND

As electric currents flowing through electronic components used inautomobiles and industrial machines increase, the operating temperaturesof the semiconductors used inside these electronic components tend toincrease. Therefore, there is a strong demand for bonding materials thatcan resist such a high-temperature environment. Conventionally,lead-containing solder that can maintain its strength at hightemperatures has been used. However, with the recent trend of reducingthe amount of lead used, there is a strong demand to provide a bondingmethod suitable for such a requirement.

As a candidate of the bonding method that can meet the aboverequirements, a bonding method using silver nanoparticles is recentlyreceiving attention. In this method, no lead is used, and bonding can beachieved under temperature conditions lower than those for bulk silver.For example, in one method proposed under the above circumstances, amixture of silver oxide particles and myristyl alcohol is used as abonding material (Non Patent Literature 1 and Patent Literature 1). Inanother proposed method, carboxylic acid is added to a mixture of silvernanoparticles and silver carbonate or silver oxide, and the resultantmixture is used as a bonding material (Patent Literature 2).

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Laid-Open No.    2009-267374-   Patent Literature 2: Japanese Patent Application Laid-Open No.    2009-279649

Non Patent Literature

-   Non Patent Literature 1: “Development of lead-free bonding    technology for high-temperature environment using silver oxide    particles of micrometer size,” Morita et al., Materia, Vol. 49, No.    1 (2010)

SUMMARY Technical Problem

In the bonding method using silver, some degree of pressurization mustgenerally be performed, for example, from above during bonding, asdescribed in the technology described in Non Patent Literature 1 and thelike. To apply such a technology, an apparatus that can perform at leastpressurization and heating simultaneously must be used, and thereforethis technology somewhat lacks versatility. Another problem of thistechnology is that it cannot be used for a material having only amechanical strength not high enough to resist pressurization. Therefore,if a paste that can provide an appropriate bonding strength withoutpressurization can be provided, the range of object materials to whichthe technology can be applied is expected to be significantly increased.

A bonded product is formed in an oxidative atmosphere containing oxygen,for example, at least in the air. Therefore, it is feared that silveroxide, which may adversely affect the bonding strength, is formed in theinterface portion. Such an influence may be significant particularly onfine bonded products. Therefore, if a bonding material can be providedwhich can provide a sufficient bonding strength in an inert atmospheresuch as a nitrogen atmosphere in which the above-described influence canbe eliminated, the range of use fields of the paste and the range ofpotential use of the paste are expected to be significantly increased.

Accordingly, the present invention provides a bonding material that canform a bonded product in nitrogen and can provide a bonding strengthhigh enough for practical use without pressurization and heat treatmentat high temperatures.

Solution to Problems

The present inventors have made extensive studies to solve the aboveproblems and found that the following bonding material can provide astrength high enough for practical use even when a bonded product isformed in an acting environment that has not been usable. Thus, theinvention has been completed.

More specifically, the foregoing problems can be solved by providing abonding material configured to include: silver nanoparticles having anaverage primary particle diameter of 1 to 200 nm and coated with anorganic material having 8 or less carbon atoms; a flux component havingat least two carboxyl groups; and a dispersion medium.

The bonding material having the above-described configuration may beconfigured to further include silver particles having an averageparticle diameter of 0.5 μm or larger and 3.0 μm or smaller.

More preferably, in the bonding material, the flux component has anether bond and a dicarboxylic acid structure.

Particularly, in the bonding material, the flux component may beoxydiacetic acid having an ether linkage and a dicarboxylic acidstructure.

Particularly, in the bonding material including the flux component, thenumber of carbon atoms of the organic material with which the surfacesof the silver nanoparticles are coated may be 6.

In the bonding material including the silver nanoparticles and the fluxcomponent described above, the dispersion medium constituting thebonding material may be a polar material.

The technical content provided by the present invention is a bondingmethod that uses a bonding material having any of the above-describedconfigurations. More specifically, the bonding method uses silvernanoparticles, and particularly the bonding material used contains aflux component. A material having a dicarboxylic acid structure havingat least two carboxyl groups is used as the flux component.

Particularly, the bonding method is characterized in that the fluxcomponent used has the dicarboxylic acid structure having at least twocarboxyl groups and also has an ether linkage.

In the bonding method, a bonded product bonded using the above-describedbonding material may be formed in an inert gas atmosphere such as anitrogen atmosphere.

In the bonding method, the bonding may be performed under heating at500° C. (773K) or lower.

Advantageous Effects of Invention

The use of the bonding material disclosed in the present inventionallows a bonded product having a bonding strength high enough forpractical use to be formed even in a nitrogen environment. In addition,a bonded product having a bonding strength equivalent to that obtainedby using conventionally used solder can be provided even withoutpressurization during heating.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a TG diagram of each of a bonding material according to thepresent invention, a raw material powder, an additive, and a surfacecoating material, as measured at a sweep rate of 10° C./min. Thevertical axis represents the relative value of the amount of reductionthat is different from the actual value.

FIG. 2 is a graph showing the amount of sorbic acid (M/Z=97) detected inthe range of 50 to 250° C. in an inert atmosphere for each of a pasteaccording to the present invention and a paste containing no additive.

FIG. 3 is an SEM image of a fired product according to Example 1.

FIG. 4 is an SEM image of a fired product according to Example 2.

FIG. 5 is an SEM image of a fired product according to ComparativeExample 1.

FIG. 6 is an SEM image of a fired product according to ComparativeExample 2.

DESCRIPTION OF EMBODIMENTS <Silver Nanoparticles>

The silver nanoparticles used in the present invention have an averageprimary particle diameter of 1 to 200 nm, preferably 1 to 150 nm, andmore preferably 10 to 100 nm as computed from a transmission electronmicroscope (TEM) photograph. The use of the particles having such aparticle diameter allows a bonded product having a high bonding strengthto be formed.

In the evaluation using the transmission electron microscope, 2 parts bymass of washed metal nanoparticles is added to a mixed solution of 96parts by mass of cyclohexane and 2 parts by mass of oleic acid and aredispersed using ultrasonic waves. Then the dispersion is dropped onto aCu microgrid with a support film and then dried to obtain a TEMspecimen. The particles on the produced microgrid are observed under thetransmission electron microscope (type JEM-100CXMark-II, product of JEOLLtd.) at an acceleration voltage of 100 kV, and a bright field image ofthe particles is taken at a magnification of 300,000×.

The diameters of the particles in the obtained photograph may bedirectly measured using, for example, a vernier caliper or may becomputed using image software. The average primary particle diameter iscomputed as the number average of the diameters of at least 200independent particles in the TEM photograph.

The silver nanoparticles used in the present invention have the particlediameter described above and are coated with an organic material. Anorganic material having 8 or less carbon atoms can be preferably used.An organic material having 8 or less carbon atoms and having at leastone carboxyl group can be particularly preferably used. Specificexamples of such an organic material include, but not limited to:saturated fatty acids such as octanoic acid (caprylic acid), heptanoicacid (enanthic acid), hexanoic acid (caproic acid), pentanoic acid(valeric acid), butanoic acid (butyric acid), and propanoic acid(propionic acid); dicarboxylic acids such as oxalic acid, malonic acid,methylmalonic acid, ethylmalonic acid, succinic acid, methylsuccinicacid, ethylsuccinic acid, phenylsuccinic acid, glutaric acid, adipicacid, pimelic acid, and suberic acid; and unsaturated fatty acids suchas sorbic acid and maleic acid.

When the surfaces of the silver nanoparticles are coated with one of theabove organic materials, particularly hexanoic acid, heptanoic acid,adipic acid, sorbic acid, or malonic acid, the particles can be easilyproduced and provided in a powder form. Such a powder form is preferablebecause the provided particles can be easily mixed when a bondingmaterial is formed as in the present invention. The particles havingsurfaces coated with such a material aggregate with the form of primaryparticles being maintained and can be easily collected. The aggregatedclusters obtained have a diameter equal to or larger than 2.5 μm whichcan be collected at least using JISP-3801 No. 5C. However, this diameterof the aggregated (secondary) particles merely means that the particlescan be separated by filtration. More specifically, the diameter of theaggregated particles is different from the average particle diameter(D50) of the silver particles (if the D50 value is used, the number ofaggregated clusters that pass through the filter paper is large, andtherefore the collection efficiency is low. However, the particlesaccording to the present invention do not pass through the filter paper,and the filtrate obtained is clear. Therefore, it can be construed thatthe aggregated clusters have a size of at least about 2.5 μm, but thisis not an average value). When drying operation at low temperatures(less than 100° C.) is added to the method, dry particles can becollected. Therefore, to design a bonding material, particles coatedwith molecules that can provide the above size are preferably used.

Silver nanoparticles coated with a plurality of organic materials may beused, or different types of silver nanoparticles having differentaverage primary particle diameters may be used.

<Silver Particles>

When silver particles of the order of submicrons are added in additionto the above-described silver nanoparticles, the bonding strength can beimproved. More specifically, it is preferable to use silver particleshaving an average particle diameter of 0.5 μm or lager. In the presentdescription, the average particle diameter is computed using a laserdiffraction method. More specifically, 0.3 g of sample silver particlesis added to 50 mL of isopropyl alcohol and dispersed for 5 minutes usingan ultrasonic cleaner at a power of 50 W. Then the D50 value (cumulative50 mass particle diameter) is measured by the laser diffraction methodusing a microtrac particle size distribution analyzer (9320-X100, aproduct of Honeywell-NIKKISO Co., Ltd.) and is used as the averageparticle diameter. The measured average particle diameter is in therange of 0.5 to 3.0 μm, preferably 0.5 to 2.5 μm, and more preferably0.5 to 2.0 μm. When such particles are also used, a bonded product witha high bonding strength can be provided.

<Dispersion Medium>

In the paste-like bonding material according to the present invention,the silver nanoparticles are dispersed in a dispersion medium. Thedispersion medium used is preferably a polar solvent because it has alow vapor pressure and can be easily handled.

Specific examples of the dispersion medium used include, water,alcohols, polyols, glycol ethers, 1-methyl pyrrolidinone, pyridine,octanediol, terpineol, butyl carbitol, butyl carbitol acetate, texanol,phenoxypropanol, diethylene glycol monobutyl ether, diethylene glycolmonobutyl ether acetate, γ-butyrolactone, ethylene glycol monomethylether acetate, ethylene glycol monoethyl ether acetate, methoxy butylacetate, methoxy propyl acetate, diethylene glycol monoethyl etheracetate, ethyl lactate, and 1-octanol.

If necessary, a material that reduces sintering temperatures andfacilitates adhesion may be added to the dispersion. Such an additivemay have a viscosity adjusting function. The additive added to thedispersion may be a water-solubilizable resin or a water-dispersibleresin. Specific examples of such an additive include acrylic resins,maleic resins, fumaric resins, high-acid value resins of styrene-maleicacid copolymer resins, polyester resins, polyolefin resins, phenoxyresins, polyimide resins, polyamide resins, vinyl acetate-basedemulsions, acrylic emulsions, synthetic rubber latex, epoxy resins,phenolic resins, DAP resins, urethane resins, fluorocarbon resins,silicone resins, and ethyl cellulose, polyvinyl alcohol. Examples ofinorganic binders include silica sol, alumna sol, zirconia sol, andtitania sol. However, the addition of an excessive amount of such aresin is not preferred because the purity of the metal is reduced. Theamount is preferably several parts by weight based on the total metalamount.

Specific names of the additives are listed below. However, the use ofadditives other than those in the following list is not exclusive, solong as they have the above-described properties. Examples of theacrylic resins include BR-102 resin and the like, (products ofMITSUBISHI RAYON Co., Ltd.) and ARUFON UC-3000 resin and the like(products of Toagosei Co., Ltd.). Examples of the polyester resinsinclude VYLON 220 and the like, (products of Toyobo Co., Ltd.) andMALKYD No. 1 and the like (products of Arakawa Chemical Industries,Ltd.). Examples of the epoxy resins include ADEKA resin EP-4088S and thelike, (products of Adeka Corporation) and 871 and the like (products ofJapan Epoxy Resins Co., Ltd.). Examples of the phenolic resins includeRESITOP PL-4348 and the like (products of Gunei Chemical Industry Co.,Ltd.). Examples of the phenoxy resins include 1256 and the like,(products of Japan Epoxy Resins Co., Ltd.) and TAMANOL 340 and the like(products of Arakawa Chemical Industries, Ltd.). Examples of the DAPresins include DAP A and the like (products of Daiso Co., Ltd.).Examples of the urethane resins include MILLIONATE MS-50 and the like(products of Nippon Polyurethane Industry Co., Ltd.). Examples of theethyl cellulose include ETHOCEL STANDARD 4 and the like (products ofNisshin & Co., Ltd.). Examples of the polyvinyl alcohol include RS-1713and the like (products of Kuraray Co., Ltd.).

<Flux Component>

In addition to the above-described components, an organic material usedas a flux component is added to the bonding material according to thepresent invention. More specifically, dicarboxylic acid having at leasttwo carboxyl groups, more preferably dicarboxylic acid having an etherlinkage and at least two carboxyl groups, may be selected. The additionof the selected material having such a configuration allows the silvernanoparticles coated with the organic material to be converted to bulksilver even by heat treatment at relatively low temperatures innitrogen.

Preferably, also the flux component is decomposed into as simpleconfigurations as possible (more specifically, finally into carbondioxide, water, and the like). Therefore, it is preferable that the fluxcomponent be an organic material composed only of elements such ascarbon, hydrogen, and oxygen. The decomposition temperature of the fluxcomponent is preferably at least equal to or lower than the presettemperature of the heat treatment. From the viewpoint of decomposition,the molecular constitution of the flux component has a molecular weightof 1,000 or lower, preferably 500 or lower, and more preferably 300 orlower.

Examples of the flux component include glycolic acid having adicarboxylic acid structure. However, excessively large molecules arenot preferred because high temperature is required for decomposition.Preferably, the decomposition temperature is at least lower than thepreset temperature of main firing. More specifically, a flux componenthaving a decomposition temperature of 500° C. or lower and morepreferably 300° C. or lower is selected. However, when a flux componentthat is directly vaporized without decomposition is used, the“decomposition temperature” is read as “evaporation (vaporization)temperature.”

More specifically, the total number of carbon atoms in the structure isat most 15 or less and preferably 10 or less. A structure having such anumber of carbon atoms can be decomposed or vaporized at relatively lowtemperatures around bonding temperature.

<Dispersant>

A dispersant that allows the silver nanoparticle powder to be welldispersed may be added to the paste according to the present invention.The use of such a dispersant allows the independence of the particles inthe paste to be ensured and also allows the reactivity between the fluxcomponent and the silver nanoparticles to be increased during theirreaction, so that a uniform bonded product can be formed at lowertemperatures. Any commercial general-purpose dispersant can be used solong as it has an affinity for the particle surfaces and an affinity forthe dispersion medium. Only one type of dispersant may be used, or aplurality of types may be used. The amount added of the dispersant ispreferably 10 percent by mass or less based on the amount of the entirepaste, preferably 5 percent by mass or less, and more preferably 3percent by mass or less.

Representative examples of the dispersant having the above-describedproperties include: low-molecular weight anionic compounds such as fattyacid salts (soap), α-sulfo fatty acid ester salts (MES), alkyl benzenesulfonates (ABS), linear alkyl benzene sulfonates (LAS), alkyl sulfates(AS), alkyl ether sulfates (AES), and triethanol alkyl sulfates;low-molecular weight nonionic compounds such as fatty acid ethanolamides, polyoxyethylene alkyl ethers (AE), polyoxyethylene alkyl phenylethers (APE), sorbitol, and sorbitan; low-molecular weight cationiccompounds such as alkyl trimethyl ammonium salts, dialkyl dimethylammonium chlorides, and alkyl pyridinium chlorides; low-molecular weightamphoteric compounds such as alkyl carboxyl betaines, sulfobetaine, andlecithin; high-molecular weight aqueous dispersants represented bynaphthalene sulfonate formalin condensates, polystyrene sulfonates,polyacrylates, salts of copolymers of vinyl compounds and carboxylicacid-based monomers, carboxymethyl cellulose, polyvinyl alcohols, andthe like; high-molecular weight nonaqueous dispersants such as partialalkyl-esters of polyacrylic acid and polyalkylene polyamines; andhigh-molecular weight cationic dispersants such as polyethyleneimine andaminoalkyl methacrylate copolymers. However, dispersants havingstructures other than those exemplified above are not excluded, so longas they can be preferably applied to the particles of the invention.

Specific names of the known dispersants are listed below. However, theuse of any dispersant other than those in the following list is notexclusive so long as it has the above-described properties. Examples ofthe specific names include BEAULIGHT LCA-H, LCA-25NH, and the like(products of Sanyo Chemical Industries, Ltd.), FLOWLEN DOPA-15B and thelike (products of Kyoeisha Chemical Co., Ltd.), SOLPLUS AX5, SOLSPERSE9000, SOLTHIX 250, and the like (products of The Lubrizol Corporation),EFKA4008 and the like (products of EFKA Additives B.V.), AJISPER PAllland the like (products of Ajinomoto Fine-Techno Co., Inc.),TEXAPHOR-UV21 and the like (products of Cognis Japan), Disper BYK2020,BYK220S, and the like (products of BYK Japan KK), DISPARLON 1751N,HIPLAAD ED-152, and the like (products of Kusumoto Chemicals, Ltd.),FTX-207S, FTERGENT 212P, and the like (products of NEOS CompanyLimited), AS-1100 and the like (products of Toagosei Co., Ltd.), KAOCER2000, KDH-154, MX-2045L, HOMOGENOL L-18, RHEODOL SP-010V, and the like(products of Kao Corporation), EPAN U103, SHALLOL DC902B, NOIGEN EA-167,PLYSURF A219B, and the like (products of Dai-Ichi Kogyo Seiyaku Co.,Ltd.), MEGAFAC F-477 and the like (products of DIC corporation), SILFACESAG503A, DYNOL 604, and the like (products of Nissin Chemical Co.,Ltd.), SN SPERSE 2180, SN LEVELER S-906, and the like (products of SanNopco Limited), and S-386 and the like (products of AGC Seimi ChemicalCo., Ltd.).

When the formed bonding material has an appropriate viscosity, it can beeasily applied to a bonding area. According to the studies by thepresent inventors, the viscosity is about 10 to about 250 Pa·s at roomtemperature, preferably about 10 to about 100 Pa·s, and more preferablyabout 10 to about 50 Pa·s. The viscosity value is a value under theconditions of 25° C., 5 rpm, and C(corn) 35/2.

<Production of Bonding Material (Paste)>

The bonding material according to the present invention is providedthrough the following production method and the like For example, silvernanoparticles obtained by the method described in Japanese Patent No.4344001 are used. The obtained silver nanoparticles, the flux componenthaving the above-described properties, and, if necessary, the dispersantare added to the above-described polar solvent. Then the mixture isintroduced in a kneading-degassing apparatus to form a kneaded productof the above components. If necessary, the kneaded product is subjectedto mechanical dispersion treatment to form a paste.

Any well-known method can be used for the mechanical dispersiontreatment, so long as the particles are not significantly reformed.Specific examples of the mechanical dispersion treatment includeultrasonic dispersion, a dispersion mill, a triple roll mill, a ballmill, a bead mill, a biaxial kneader, and a planetary mixer. These maybe used alone or in combination of two or more.

<Formation of Bonded Product>

A bonded portion is formed as follows. The bonding material is appliedto a thickness of about 20 to about 200 μm using, for example, a metalmask, a dispenser, or screen printing. Then an object to be bonded isattached, and then the bonding material is metalized by heat treatment.The present paste can be metalized by heat treatment in nitrogen and canalso be metalized by heat treatment in the air.

The use of the bonding material of the present invention allows a bondedproduct to be formed without pressurizing the objects to be bonded.However, this does not exclude the step of pressurization. The additionof the step of pressurizing the objects to be bonded is preferredbecause gases generated from the silver nanoparticles or the dispersionmedium can be removed in a more efficient manner.

Generally, when pressurization is performed, the higher the pressure,the more preferred. However, a pressure higher than necessary is notrequired. Pressurization at about 5 MPa is sufficient because asufficiently high bonding strength can be obtained.

<Preliminary Firing Step>

Preferably, metallization by multi-stage heat treatment is performed toform a bonded product using the paste of the present invention. Morespecifically, the following steps are used. A first firing step(preliminary firing step) is performed for the purpose of evaporatingand removing the solvent added to the bonding material. However, if theheat treatment is performed at excessively high temperatures, not onlythe solvent but also the organic material forming the surfaces of thesilver nanoparticles may be removed. This is not preferred because aharmful influence such as a reduction in bonding strength is exerted.More specifically, the heat treatment is performed preferably at atemperature lower than the decomposition temperature of the silvernanoparticles.

The decomposition temperature of the silver nanoparticles can greatlyvary depending on the organic material that coats the surfaces of theparticles, the dispersion medium, and the additives. Therefore, it ispreferable to determine the thermal properties of the bonding materialby, for example, TG measurement in advance. It is generally preferablethat the decomposition temperature be set to a temperature lower byabout 50 to about 400° C. than the preset temperature of the mainfiring. The time required for the preliminary firing depends on the areato be bonded. It is sufficient that the firing time is about 10 minutes.In some cases, heating for about 30 seconds is sufficient.

<Main Firing Step>

After the preliminary firing, a main firing step is performed tocompletely metalize the paste. A temperature rising step may be providedbetween the preliminary firing step and the main firing step. Thetemperature rising rate in the temperature rising step is in the rangeof 0.5 to 10° C./sec and more preferably 0.5 to 5° C./sec.

During main firing, the temperature is maintained at 150° C. or higherand 500° C. or lower for 60 minutes or shorter or 30 minutes or shorter.In this process, pressurization at 10 MPa or lower may be performed asneeded.

The crystals in the thus-obtained bonded product are very large eventhough the crystals have grown in an inert atmosphere. Specific valuesof the diameters of the crystallites on the (111) plane of Ag that arecomputed from the half width of X-rays are 65 nm or larger even afterheat treatment at 250° C. for 10 minutes. The larger these values, themore preferred, because such large values indicate that less crystalgrain boundaries are formed between the particles. The diameter of thecrystallites is more preferably 67 nm or larger and still morepreferably 70 nm or larger.

EXAMPLES Synthesis of Silver Nanoparticles

13.4 g of silver nitrate (a product of Toyokagaku Co., Ltd.) wasdissolved in 72.1 g of pure water in a 500 mL beaker to prepare a silversolution.

Next, a 5 L beaker was charged with 1.34 L of pure water, and the purewater was bubbled with nitrogen for 30 minutes to remove dissolvedoxygen and was then heated to 60° C. Then 17.9 g of sorbic acid (aproduct of Wako Pure Chemical Industries, Ltd.) was added. Next, 2.82 gof 28% ammonia water (a product of Wako Pure Chemical Industries, Ltd.)was added to adjust pH. In the following Examples and ComparativeExamples, the addition of ammonia water starts a reaction. Five minutesafter the start of the reaction, 5.96 g of hydrazine hydrate (a productof Otsuka Chemical Co., Ltd., purity: 80%) was added to the mixtureunder stirring.

Nine minutes after the start of the reaction, the silver solution wasadded and allowed to react. Then the mixture was aged for 30 minutes toform silver nanoparticles coated with sorbic acid. The silvernanoparticles were filtrated using a No. 5C paper filter and washed withpure water to obtain aggregated clusters of the silver nanoparticles.The aggregated clusters were dried in a vacuum dryer under theconditions of 80° C. for 12 hours to obtain a dry powder of theaggregated clusters of the silver nanoparticles.

90.0 g of the dry powder of the aggregated clusters of the sorbicacid-coated silver nanoparticles (average primary particle diameter: 60nm) obtained by the above-described method was mixed with 8.80 g ofterpineol (a mixture of structural isomers, a product of Wako PureChemical Industries, Ltd.), 1.00 g (1.0% based on the total weight of apaste) of BEAULIGHT LCA-25NH used as a wetting dispersant (a product ofSanyo Chemical Industries, Ltd.), and 0.20 g (0.2% based on the totalweight of the paste) of oxydiacetic acid. The mixture was kneaded for 30seconds using a kneading-degassing apparatus (Type V-mini 300, a productof EME Corporation) (kneading conditions/Revolution: 1,400 rpm,Rotation: 700 rpm) and then subjected to a triple roll mill (type EXAKT80S, product of EXAKT Apparatebau) five times to produce a bondingmaterial paste. Then the obtained bonding material was applied to asubstrate by a printing method. More specifically, the bonding materialwas applied to a silver-plated copper substrate using a metal squeegeeby manual printing under the following conditions: metal mask (maskthickness: 50 μmt), pattern (2 mm), thickness (50 μm). The mixing ratioand the like are shown in Tables 1 and 2.

A chip (2 mm square silver-plated cupper substrate with a thickness of 2mm) was mounted on the coated surface. The obtained chip-mounted productwas heated in a furnace (desktop lamp heating unit, type MILA-5000, aproduct of ULVAC-RIKO, Inc.) at 100° C. in a nitrogen atmosphere (oxygenconcentration: 50 ppm or lower) for 10 minutes to remove the solventcomponent in the paste (preliminary firing). To examine the specificresistance and sintered state of the fired film, a specimen with no chipmounted on the bonding material was simultaneously prepared by printingonly the bonding material onto a substrate and then firing the coatedsubstrate.

The specimens subjected to the preliminary firing were then heated to250° C. at a temperature rising rate of 1° C./sec and subjected to heattreatment at 250° C. for 10 minutes (main firing), and a bonded productwas thereby obtained. In this Example, no pressure was applied duringthe preliminary firing step and the main firing step. The testconditions are shown in Table 3.

The bonding strength of the obtained bonded product was examined. Morespecifically, the bonding strength was examined according to a methoddescribed in JISZ-03918-5:2003 “Lead-free solder test methods, Part 5,Tensile and shear test methods for solder joints.” In this method, abonding object (a chip) bonded to a substrate is pushed in a horizontaldirection, and a pushing force that causes the bonding surfacewithstanding the force to be eventually broken is measured. In thisExample, the test was performed using a bond tester (series 4000, aproduct of DAGE). The measurement was performed at room temperature witha shear height of 150 μm, and a test rate of 5 mm/min. The specificresistance of the fired film was measured by the four-probe method.

The results showed that the shear strength in Example 1 was 63.5 MPa.The specific resistance value of the fired film was 3.11 μΩ·cm, and thefired film was found to have a very high conductivity. The SEM image ofthe obtained fired film is shown in FIG. 3. As is clear from thephotograph, although the firing had been performed in nitrogen,sintering of the particles had proceeded to the extent that the shapesof the particles were not distinguishable. The above results suggestthat the sintering of the particles had proceeded to a considerableextent. In the shear test method, the force (N) when the bonding surfacewas broken was directly measured, and this value depends on the bondingarea. Therefore, to obtain a normalized value, the measured force uponbreakage was divided by the bonding area (2 (mm)×2 (mm)=4 mm² in thiscase), and the resultant value (MPa) was defined as a shear strength.The same applies to samples described later.

The size of the crystallites in the film used for the SEM measurement inFIG. 3 was examined using an X-ray diffraction apparatus. In the methodof measuring the size of the crystallites in the present invention, themeasurement was performed six times on a (111) plane over the range of40 to 50°/20 in RINT 2100 (a product of Rigaku Corporation) using a Coradiation source (40 kV/30 mA) to obtain cumulative results. Thecrystallite diameter was computed from the half width β obtained by themeasurement using the Scherrer equation represented by the followingequation (1).

Dhkl=(K·λ)/(β·cos θ)  (1)

The variables in the equation (1) are as follows.

D: crystallite diameter (nm)

λ: wavelength of measurement X-rays (nm)

β: spread of diffraction width by crystallites

θ: Bragg angle of diffraction

K: Scherrer constant

1.79 was substituted for the wavelength X of the measurement X-rays inthe equation (1), and 0.94 was substituted for the Scherrer constant K.The determined crystallite diameter was 76.33 nm, and the growth of thecrystal grains were found to proceed to a considerable extent.

The amount of sorbic acid (M/Z=97) detected in the range of 50 to 250°C. in an inert atmosphere was determined using TG-MS apparatuses. In themeasurement method used, TG measurement was performed using Thermo PlusTG8120 (a product of Rigaku Corporation) in a helium gas flow of 100mL/min under the temperature rising condition of 10° C./min. MSmeasurement was performed using a mass spectrometer QP-5000 compositesystem (a product of Shimadzu Corporation) with an inlet temperature of250° C., and an interface temperature of 300° C. An EI method with 70 eVwas used as an ionization method, and the mass range of scan was 10 to500.

The results are shown in FIG. 2. The horizontal axis representstemperature, and the vertical axis represents the amount detected(counts). In the paste according to this Example (Example 1), a largeamount of sorbic acid adhering to the surfaces of the silvernanoparticles was observed in the range of 100 to 150° C. However, inthe results when no additive was used, a large amount of sorbic acid wasobserved in a relatively wide range of 100 to 200° C., and a localmaximum was found in the range of 150 to 200° C.

Example 2

A bonded product and a fired film were formed by the same procedure asin Example 1 except that the conditions for the main firing were 350° C.for 5 minutes. The shear strength was 56.2 MPa and found to be veryhigh. The specific resistance value of the fired film was 2.4 μΩ·cm, andthe fired film was found to have a very high conductivity. The SEM imageof the obtained fired film is shown in FIG. 4. Although the firing wasperformed in nitrogen, sintering of the particles had proceeded to theextent that the shapes of the particles were not distinguishable. Thissuggests that the sintering of the particles had proceeded to aconsiderable extent. The mixing ratio and the like are shown in Tables 1and 2. The test conditions are shown in Table 3. The crystallitediameter determined as in Example 1 was 73.58 nm, and the growth of thecrystal grains were found to proceed to a considerable extent, as in theresults shown in the photograph.

Example 3

Example 1 was repeated using the same mixing ratio as in Example 1except that 90.0 g of the nanoparticles (coated with sorbic acid) usedas the metal component in Example 1 was replaced with a mixture of 45.0g of spherical submicron silver particles (a product of DOWA ElectronicsMaterials Co., Ltd., average particle diameter (D50 value): 1.0 μm) and45.0 g of sorbic acid-coated silver nanoparticles obtained by the methoddescribed in Example 1. The total amount of the metal components was90.0 g which was the same as that in Example 1. The results ofevaluation of the obtained bonding material are shown in Table 3.

Example 4

Example 2 was repeated using the same mixing ratio as in Example 2except that 90.0 g of the nanoparticles (coated with sorbic acid) usedas the metal component in Example 2 was replaced with a mixture of 45.0g of spherical submicron silver particles (a product of DOWA ElectronicsMaterials Co., Ltd., average particle diameter (D50 value): 1.0 μm) and45.0 g of sorbic acid-coated silver nanoparticles obtained by the methoddescribed in Example 1. The total amount of the metal components was90.0 g which was the same as that in Example 2. The results ofevaluation of the obtained bonding material are shown in Table 3.

Example 5

Example 2 was repeated using the same mixing ratio as in Example 2except that 90.0 g of the nanoparticles (coated with sorbic acid) usedas the metal component in Example 2 was replaced with a mixture of 22.5g of spherical submicron silver particles (a product of DOWA ElectronicsMaterials Co., Ltd., average particle diameter (D50 value): 1.0 μm) and67.5 g of sorbic acid-coated silver nanoparticles obtained by the methoddescribed in Example 1. The total amount of the metal components was90.0 g which was the same as that in Example 2. The results ofevaluation of the obtained bonding material are shown in Table 3.

Example 6

Example 2 was repeated using the same mixing ratio as in Example 2except that 90.0 g of the nanoparticles (coated with sorbic acid) usedas the metal component in Example 2 was replaced with a mixture of 67.5g of spherical submicron silver particles (a product of DOWA ElectronicsMaterials Co., Ltd., average particle diameter (D50 value): 1.0 μm) and22.5 g of sorbic acid-coated silver nanoparticles obtained by the methoddescribed in Example 1. The total amount of the metal components was90.0 g which was the same as that in Example 2. The results ofevaluation of the obtained bonding material are shown in Table 3.

Example 7

Example 4 was repeated using the same mixing ratio as in Example 4except that 0.2 g oxydiacetic acid used in Example 4 was replaced with0.1 g of oxydiacetic acid. The results of evaluation of the obtainedbonding material are shown in Table 3.

Example 8

Example 7 was repeated except that the substrate plated with silver inExample 7 was replaced with a substrate with a solid copper surface. Theresults of evaluation of the obtained bonding material are shown inTable 3.

Example 9

Example 4 was repeated except that the substrate plated with silver inExample 4 was replaced with a substrate with a solid copper surface. Theresults of evaluation of the obtained bonding material are shown inTable 3.

Example 10

Example 4 was repeated except that 0.2 g of oxydiacetic acid used as theflux component in Example 4 was replaced with 0.1 g of malonic acid. Theresults of evaluation of the obtained bonding material are shown inTable 3.

Comparative Example 1

Tests were performed as in Example 1 except that a bonding material towhich no oxydiacetic acid was added was produced. The maximum value ofthe shear strength was 4.0 MPa, and the average value was 2.7 MPa (theaverage of 5 points). Therefore, enough bonding strength was notobtained. In one of the samples, the bonding strength could not becomputed. The specific resistance of the fired film was 7.77 μΩ·cm andwas higher than the specific resistance in Example 1. The SEM image ofthe obtained fired product is shown in FIG. 5. Since the firing had beenperformed in nitrogen, sintering of the particles had not sufficientlyproceeded, and the shapes of the individual particles were found to bedistinguishable. The mixing ratio and the like are shown in Tables 1 and2. The test conditions are shown in Table 3. The crystallite diameterdetermined as in Example 1 was 57.92 nm, and the growth of the crystalswas found to proceed to a lesser extent than that in the Examples, as inthe results shown in the photograph.

Comparative Example 2

Tests were performed as in Example 2 except that a bonding material towhich no oxydiacetic acid was added was produced. The maximum value ofthe shear strength was 13.2 MPa, and the average value was 9.5 MPa (theaverage of 5 points). Therefore, enough bonding strength was notobtained. The specific resistance of the fired film was 4.20 μΩ·cm andwas higher than the specific resistance in Example 1. The SEM image ofthe obtained fired product is shown in FIG. 6. Since the firing had beenperformed in nitrogen, sintering of the particles had not sufficientlyproceeded, although the degree of sintering was higher than that inComparative Example 1. The shapes of the individual particles were foundto be distinguishable. The mixing ratio and the like are shown in Tables1 and 2. The test conditions are shown in Table 3. The crystallitediameter determined as in Example 1 was 62.68 nm, and this also showsthat the growth of the crystals had proceeded to a lesser extent thanthat in the Examples, as in the results shown in the photograph.

Reference Example 1

A bonding object was bonded using a commercially availablehigh-temperature solder paste (SN515 RMA A M Q M-293T, a product ofNIHON SUPERIOR Co., Ltd.). The bonding was performed by coating asubstrate with the paste in the air, placing the bonding object on thepaste, pressurizing the object at 0.5 N, drying the paste at 150° C. for2 minutes, and heating at 350° C. for 40 seconds to metalize the bondingmaterial in the metal bonding surface. The bonding strength of theobtained bonded product was 36.7 MPa.

Reference Example 2

A bonding object was bonded using a commercially available lead-freesolder paste (M705-K2-V, a product of Senju Metal Industry Co., Ltd.).In the bonding method used, a substrate was coated with the paste in theair, and the bonding object was placed on the paste. Then the object waspressurized at 0.5 N. The paste was dried at 150° C. for 2 minutes andheated at 250° C. for 40 seconds to metalize the bonding material in themetal bonding surface. The bonding strength of the obtained bondedproduct was 40.0 MPa.

TABLE 1 NANOPARTICLES MICROPARTICLES COATING AVERAGE PRIMARY AMOUNTAVERAGE PARTICLE AMOUNT NANOPARTICLES MATERIAL PARTICLE DIAMETER ADDEDSHAPE DIAMETER ADDED MICROPARTICLES EXAMPLE 1 SORBIC ACID 60 nm 90.0 g —— — — EXAMPLE 2 SORBIC ACID 60 nm 90.0 g — — — — EXAMPLE 3 SORBIC ACID60 nm 45.0 g SPHERICAL 1.0 μm 45.0 g 1.0 EXAMPLE 4 SORBIC ACID 60 nm45.0 g SPHERICAL 1.0 μm 45.0 g 1.0 EXAMPLE 5 SORBIC ACID 60 nm 67.5 gSPHERICAL 1.0 μm 22.5 g 3.0 EXAMPLE 6 SORBIC ACID 60 nm 22.5 g SPHERICAL1.0 μm 67.5 g 0.3 EXAMPLE 7 SORBIC ACID 60 nm 45.0 g SPHERICAL 1.0 μm45.0 g 1.0 EXAMPLE 8 SORBIC ACID 60 nm 45.0 g SPHERICAL 1.0 μm 45.0 g1.0 EXAMPLE 9 SORBIC ACID 60 nm 45.0 g SPHERICAL 1.0 μm 45.0 g 1.0EXAMPLE 10 SORBIC ACID 60 nm 45.0 g SPHERICAL 1.0 μm 45.0 g 1.0COMPARATIVE SORBIC ACID 60 nm 90.0 g — — — — EXAMPLE 1 COMPARATIVESORBIC ACID 60 nm 90.0 g — — — — EXAMPLE 2

TABLE 2 FLUX DISPERSANT DISPERSION MEDIUM COMPONENT AMOUNT ADDED NAMEAMOUNT ADDED COMPONENT AMOUNT ADDED EXAMPLE 1 OXYDIACETIC ACID 0.2 gBEAULIGHT 1.0 g TERPINEOL 8.8 g EXAMPLE 2 OXYDIACETIC ACID 0.2 gBEAULIGHT 1.0 g TERPINEOL 8.8 g EXAMPLE 3 OXYDIACETIC ACID 0.2 gBEAULIGHT 1.0 g TERPINEOL 8.8 g EXAMPLE 4 OXYDIACETIC ACID 0.2 gBEAULIGHT 1.0 g TERPINEOL 8.8 g EXAMPLE 5 OXYDIACETIC ACID 0.2 gBEAULIGHT 1.0 g TERPINEOL 8.8 g EXAMPLE 6 OXYDIACETIC ACID 0.2 gBEAULIGHT 1.0 g TERPINEOL 8.8 g EXAMPLE 7 OXYDIACETIC ACID 0.1 gBEAULIGHT 1.0 g TERPINEOL 8.9 g EXAMPLE 8 OXYDIACETIC ACID 0.1 gBEAULIGHT 1.0 g TERPINEOL 8.9 g EXAMPLE 9 OXYDIACETIC ACID 0.2 gBEAULIGHT 1.0 g TERPINEOL 8.8 g EXAMPLE 10 MALONIC ACID 0.1 g BEAULIGHT1.0 g TERPINEOL 8.9 g COMPARATIVE BEAULIGHT 1.0 g TERPINEOL 9.0 gEXAMPLE 1 COMPARATIVE BEAULIGHT 1.0 g TERPINEOL 9.0 g EXAMPLE 2

TABLE 3 FIRING CONDITIONS EVALUATION PRELIMINARY FIRING TEMPER- MAINFIRING RESULTS SUBSTRATE FIRING ATURE FIRING SPECIFIC SILVER SOLIDATMOS- TEMPER- FIRING RISING ATMOS- TEMPER- FIRING RESIS- SHEAR PLATINGCOPPER PHERE ATURE TIME RATE PHERE ATURE TIME TANCE STRENGTH EXAM- ○NITRO- 100° C. 10 MIN- 1° C./ NITRO- 250° C. 10 MIN- 3.11 μΩ · cm 63.9MPa PLE 1 GEN UTES SECOND GEN UTES EXAM- ○ NITRO- 100° C. 10 MIN- 1° C./NITRO- 350° C. 5 MIN- 2.40 μΩ · cm 56.2 MPa PLE 2 GEN UTES SECOND GENUTES EXAM- ○ NITRO- 100° C. 10 MIN- 1° C./ NITRO- 250° C. 10 MIN- 3.70μΩ · cm 42.5 MPa PLE 3 GEN UTES SECOND GEN UTES EXAM- ○ NITRO- 100° C.10 MIN- 1° C./ NITRO- 350° C. 5 MIN- 3.60 μΩ · cm 35.6 MPa PLE 4 GENUTES SECOND GEN UTES EXAM- ○ NITRO- 100° C. 10 MIN- 1° C./ NITRO- 350°C. 5 MIN- 3.30 μΩ · cm 44.3 MPa PLE 5 GEN UTES SECOND GEN UTES EXAM- ○NITRO- 100° C. 10 MIN- 1° C./ NITRO- 350° C. 5 MIN- 7.20 μΩ · cm 30.6MPa PLE 6 GEN UTES SECOND GEN UTES EXAM- ○ NITRO- 100° C. 10 MIN- 1° C./NITRO- 350° C. 5 MIN- 2.90 μΩ · cm 56.1 MPa PLE 7 GEN UTES SECOND GENUTES EXAM- ○ NITRO- 100° C. 10 MIN- 1° C./ NITRO- 350° C. 5 MIN- — 31.6MPa PLE 8 GEN UTES SECOND GEN UTES EXAM- ○ NITRO- 100° C. 10 MIN- 1° C./NITRO- 350° C. 5 MIN- — 48.8 MPa PLE 9 GEN UTES SECOND GEN UTES EXAM- ○NITRO- 100° C. 10 MIN- 1° C./ NITRO- 350° C. 5 MIN- 3.10 μΩ · cm — PLE10 GEN UTES SECOND GEN UTES COMPAR- ○ NITRO- 100° C. 10 MIN- 1° C./NITRO- 250° C. 5 MIN- 7.77 μΩ · cm 4.0 MPa ATIVE GEN UTES SECOND GENUTES EXAM- PLE 1 COMPAR- ○ NITRO- 100° C. 10 MIN- 1° C./ NITRO- 350° C.5 MIN- 4.20 μΩ · cm 13.2 MPa ATIVE GEN UTES SECOND GEN UTES EXAM- PLE 2

The details of the reason for the above effects are not clear. However,the following reaction mechanism can be considered. The reactionmechanism will be considered with reference to the TG diagram shown inFIG. 1. The behavior of general silver nanoparticles (silvernanoparticles coated with sorbic acid in FIG. 1) is represented by acurve 10. The TG behavior of a paste having the configuration of thepresent invention is represented by a curve 20, and the curve 20 showsthat the reaction proceeds at lower temperatures. The decompositiontemperature of diglycolic acid used as the flux component added ishigher (curve 30). Therefore, it is clear that the reduction is notcaused by the diglycolic acid. Referring to the TG behavior of sorbicacid represented by alternate long and short dashed lines (curve 40),the TG behavior of the paste substantially agrees with the TG behaviorof the sorbic acid. This suggests that the sorbic acid does not adhereto the surfaces of the silver nanoparticles but is present independentlyin the paste. Therefore, it is presumed that the addition of diglycolicacid to the paste has the effect of at least removing the sorbic acidthat coats the surfaces. It is considered that this behavior allows thesilver nanoparticles to be converted to silver even in a low-reactiveenvironment in which no oxygen and the like are present around theparticles.

In addition, the use of the bonding agent according to the presentinvention allows a bonded product having a bonding strength much higherthan that obtained when a commercially available solder paste is usedfor bonding to be obtained even when firing has been performed innitrogen.

INDUSTRIAL APPLICABILITY

The bonding performed according to the present invention can be appliedto non-insulated type semiconductor devices and a bear chipmounting-assembling technology and can also be applied to a bonding stepperformed during production of power devices (IGBTs, rectifier diodes,power transistors, power MOSFETs, insulated-gate bipolar transistors,thyristors, gate turn-off thyristors, and triacs). The bonding materialcan be applied to a bonding material for glass having a chromium-treatedsurface and can be used for electrodes and frames of lighting devicesusing LEDs.

1. A bonded product obtained by applying a silver paste containingsilver nanoparticles having an average primary particle diameter of 1 to200 nm, and performing firing, wherein a diameter of a crystallite on a(111) plane of Ag when heated at 250° C. for 10 minutes in an inertatmosphere is 65 nm or larger.
 2. A bonded product obtained by applyinga silver paste containing silver nanoparticles having an average primaryparticle diameter of 1 to 200 nm and coated with an organic materialhaving 1-8 carbon atoms, a flux component of oxydiacetic acid, and adispersion medium, and performing firing, wherein a diameter of acrystallite on a (111) plane of Ag when heated at 250° C. for 10 minutesin an inert atmosphere is 65 nm or larger.